In an optical communication network, optical signals having a plurality of optical channels at individual wavelengths, called “wavelength channels”, are transmitted from one location to another, typically through a length of an optical fiber. An optical cross-connect module allows switching of optical signals from one optical fiber to another. A wavelength-selective optical cross-connect, or a reconfigurable optical add-drop module (ROADM), allows wavelength-dependent switching, that is, it allows certain wavelength channels to be switched from a first optical fiber to a second optical fiber while letting the other wavelength channels propagate in the first optical fiber, or it allows certain wavelength channels to be switched to a third optical fiber. An optical network architecture based on wavelength-selective optical switching, which is sometimes called an “agile” optical network architecture, has many attractive features due to its ability to automatically create or re-route optical paths of individual wavelength channels. It accelerates service deployment, accelerates rerouting around points of failure of an optical network, reduces capital and operating expenses for a service provider, as well as creates a future-proof topology of the network.
Of many ROADM architectures presently available, an architecture based on free-space optics and a micro-electro-mechanical system (MEMS) array or a liquid crystal (LC) array is one of the most versatile and high-performance architectures. In particular, a folded symmetrical 4-f configuration taught in U.S. Pat. No. 6,498,872 by Bouevitch et al., with an optional field-flattening optical wedge taught in U.S. Pat. No. 6,760,501 by Iyer et al., both assigned to JDS Uniphase Corporation and incorporated herein by reference, allow construction of ROADMs such as a dynamic gain equalizer module for equalizing optical power values of individual wavelength channels, a wavelength blocker module for completely blocking any subset of a full set of the wavelength channels, and a wavelength selective optical switch module for performing the abovementioned wavelength channel switching function. Among the ROADMs based on the folded 4-f configuration are: a wavelength blocker module taught in U.S. Pat. No. 7,014,326 by Danagher et al. and a multiport wavelength selective switch modules taught in U.S. Pat. Nos. 6,707,959 by Ducellier et al. and 6,810,169 by Bouevitch, both assigned to JDS Uniphase Corporation and incorporated herein by reference; and a multi-module units taught in US Patent application publication 20070242953 by Keyworth et al., incorporated herein by reference. Advantageously, the folded 4-f ROADMs have a reduced number of optical elements and reduced physical size, as compared to other existing free-space ROADM optical configurations of similar functionality. More details on optical principle of operation of the folded 4-f ROADMs can be found in the abovementioned US patent documents.
ROADMs are generally deployed at various nodes of an optical network and, therefore, they must perform reliably in harsh environments characterized by a wide range of temperature and humidity, which typically requires using a hermetic enclosure to package a ROADM. Hermetic enclosures are known. They are frequently used to package small optoelectronic and electro-optic devices such as photodiodes and optical power monitors, laser diodes, or receivers, most of which have a maximum outer dimension of half an inch or smaller. A small ceramic substrate is sometimes implemented in devices requiring a good radio-frequency (RF) electrical performance. However, these methods of hermetic packaging are not practical for a ROADM because of comparatively large optics footprint of approximately one square inch or more, and a large number of electrical feedthroughs required, from about fifty to a few hundred feedthroughs.
Due to a relatively large footprint of optics, and due to a large number of electrical connections to a MEMS or an LC optical switching engine as explained above, mechanical packaging of a ROADM represents a considerable technical challenge. As noted above, a hermetically sealed enclosure box is generally implemented to ensure the required degree of environmental stability and reliability. A flexible printed circuit board (flex-PCB) is used in the prior art to provide required electrical connections within the hermetic enclosure, while mechanically de-coupling the switching engine from an internal multi-pin hermetic electrical connector mounted on a wall of the box. It takes a long time and considerable operator skill and effort to fit all the fiber feed-throughs, flex-PCBs, and to fit and align the optical elements inside the package, which increases the cost of the assembly and reduces manufacturing yields due to an increased possibility of an accidental damage to a fiber, an optic, or a switching engine. Furthermore, a ROADM package built using this existing technology has a relatively large footprint since both the optics, the flex-PCB, and the multi-pin hermetic connector need to be accommodated inside the package. The large size of a ROADM package is highly detrimental because telecom system providers are strongly motivated to increase the element density of their circuit cards, to facilitate a decrease in the system size and cost.
It is therefore the goal of the present invention to provide a ROADM that is free from the drawbacks of a large package size, considerable complexity of assembly, lowered assembly yields, and a comparatively long assembly time.
The present invention meets the above stated goal; furthermore, advantageously and unexpectedly, it greatly improves versatility and modularity of ROADM packaging and enables a straightforward on-board ROADM electronics integration, which considerably simplifies subsequent utilization of a ROADM in an agile optical network system.